This invention relates to the data recording art, and particularly to medical imaging, where it is common to electronically view and store pictorial and other data derived from radiographic, ultrasonic, nuclear magnetic resonance or other devices capable of non-destructively inspecting the interior of a living body.
The results of such an inspection are frequently displayed, in real time, on a device such as a cathode ray tube (CRT) for interpretation by the diagnostician. They may also be preserved for future reference by the use of a recording system wherein photographic film is sequentially exposed, on demand, to a series of desired video images derived from an independent high-resolution monitor CRT fed with video signals and connected in parallel with the viewing device.
Camera systems for making permanent copies of video monitor images are described in U.S. Pat. Nos. 4,027,315 and 4,240,729, both assigned to the assignee of this application. In such camera systems, a drift in CRT luminance output can result in a loss of image detail as a result of under or over exposure of the photographic film. In order to preserve the maximum resolution of the system, the absolute luminance output and luminance range of the CRT image require limitation and stabilization to prevent exposure errors on the film. Stabilizing the maximum image luminance output at a point just below the shoulder of the film density-versus-exposure characteristic avoids over exposing the film, with an attendant loss in image detail. A similar requirement exists for stabilizing the luminance output of image details to be recorded above the toe region of the film density-versus-exposure characteristic, in order to maximize the overall resolution of the recorded image.
The detail contained between the image luminance end points is of prime interest to the medical diagnostician. By maintaining the image end points at calibrated brightness levels, detail rendition is preserved. Any variation of the end points of the range of interest raises substantial questions as to whether a complete and accurate image is available upon which to provide a proper diagnosis. Image contrast in the area of diagnostic interest is extremely important, and its stabilization with respect to time enhances the confidence of the diagnostician in later-produced images.
The portion of the density-versus-exposure characteristic lying between the toe and shoulder regions of a given photographic material, known to those skilled in the art as the brightness acceptance range of the photographic emulsion, should therefore be matched to the absolute brightness and contrast range (i.e., luminance output range) of the CRT image, and be maintained in that matched state over at least the period of time during which photographs of the images are being made. Further, it is desirable to maintain this relationship for all of the photographic materials used in recording the video images, despite their differing density-versus-exposure characteristics, and regardless of image polarity. The aforesaid related patent applications describe, respectively, an automatic and a manual apparatus for maintaining the light output of a CRT of a monitor display within the prescribed brightness acceptance limits of a photographic film. Both types of apparatus have contrast and brightness controls which are set to produce a known exposure of a photographic film. First and second synthetic signals are generated which represent opposite ends of the video signal amplitude excursion. By referring to the Dunn Density Index.TM. for a given camera shutter speed and film type, the light output of the CRT for each of these synthetic signals which lies within the film light acceptance range may be predicted. The monitor brightness and contrast are adjusted to produce these known output light levels when excited with these synthetic signals. Once the contrast and brightness levels are appropriately set, the video signal will generate a CRT image within the brightness acceptance range of the selected film type. Photographs made of these images will contain a maximum of detail as a result of matching the CRT light output range to the photographic film acceptance range.
The apparatus described in these earlier patent applications, particularly that of Ser. No. 444,546, exhibits interaction between contrast and brightness adjustments which makes calibration tedious. Further, the brightness and contrast adjustment sequence is reversed when the image polarity is changed, which can confuse the camera operator when calibrating with an image of a changed polarity. The present invention is designed to overcome these difficulties.